(1) Field of the Invention
The present invention relates to various types of boilers including a steam boiler, hot-water boiler, a waste-heat boiler, and an exhaust-gas boiler.
(2) Description of the Related Art
A boiler of a conventional multitubular boilers includes a can body constructed by arraying a number of water tubes concentrically and cylindrically between an upper header and a lower header, which are annually formed. In the above-described can body, an inside of an inner water tube row serves as a combustion chamber, and a clearance between the inner water tube row and an outer water tube row serves as a combustion gas path.
Accordingly, when combustion of fuel is performed toward the combustion chamber from a burner installed in an upper portion of the can body, combustion gas reverses in a lower portion of the combustion chamber, and passes between the inner water tube row and the outer water tube row to be exhausted to a flue from the upper portion of the can body as exhaust gas. During this process, the combustion gas performs heat exchange with water inside the respective water tubes, so that the water inside the water tubes is heated. In order to effectively perform the heat transfer to the water inside the water tubes, each of the outer water tubes (and the inner water tubes) is provided with fins to increase a heat-transfer area.
However, it needs to be considered that a combustion gas temperature decreases downstream. For example, in the can body of the above-described boiler, the temperature of the combustion gas flowing upward through the combustion gas path between the inner water tube row and the outer water tube row decreases upward. That is, a lower portion of the combustion gas path is a higher-temperature portion as compared with an upper portion thereof.
Accordingly, merely installing the fins in the same shape at equal vertical intervals in the respective water tubes without considering the above-described situation leads to an increase in heat stress occurring in the fins attached on an upstream side. Particularly, when scale (deposit of hardness in the water) adheres to insides of the water tubes, the heat transfer from the fins to the water inside the water tubes is inhibited, thereby causing a possibility of generating excessive heat stress in the fins. In this case, there is a possibility that the fins are overheated, thereby dropping off or burning out. On the other hand, pressure loss when the combustion gas enters the combustion gas path from the combustion chamber needs to be considered as well.
While, in view of these points, it can be considered that no fins are installed in the high-temperature portion, it is not preferable for efficient heat recovery not to utilize the high-temperature portion. On the other hand, if the fins in an entire region in a vertical direction excluding a small-diameter portion for communicating the inside and the outside of the inner water tube row are provided, or the fins in an entire region in the vertical direction of the outer row water tubes are provided, it may cause too much pressure loss when the combustion gas enters the combustion gas path from the combustion chamber, or may cause too high heat conductivity from the combustion gas to the respective water tubes immediately after the combustion gas turns in a lower portion of the combustion chamber.